Investments (Amortized Cost and Fair Value of Fixed Maturity Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Due in one year or less, cost or amortized cost	$ 157.9
Due after one year through five years, cost or amortized cost	1,488.6
Due after five years through ten years, cost or amortized cost	2,766.8
Due after ten years, cost or amortized cost	1,069.9
Total, cost or amortized cost	5,483.2
Fixed maturity securities available for sale, amortized cost	7,292.4	$ 6,920.8
Due in one year or less, fair value	158.1
Due after one year through five years, fair value	1,451.7
Due after five years through ten years, fair value	2,668.1
Due after ten years, fair value	944.2
Total, fair value	5,222.1
Fixed maturity securities, fair value	6,912.1	6,283.7
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	383.1
Fair Value	330.2
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	534.7
Fair Value	486.0
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	891.4
Fixed maturity securities available for sale, amortized cost	891.4	735.1
Fair Value	873.8
Fixed maturity securities, fair value	$ 873.8	$ 696.3

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